sounds like a keep problem to me,if the handle works ok when door is open then this is the case,you need keeps ajusting to work smoothly,try and do this yourself if you have a bit of knowlege,if not phone a few double glazing repair company in your area and get a few quotes,they will be cheaper than a locksmith.Good luck
shaun www.sdwupvcrepair.co.uk, February 2008